Water source rancid, so flush (4-3)
I believe the answer is:
well-off
'flush' is the definition.
(thesaurus)
'water source rancid' is the subsidiary indication.
'water source' becomes 'well' (I've seen this before).
'rancid' becomes 'off' (I've seen this before).
'well'+'off'='welloff'
'so' is the link.
(Other definitions for well-off that I've seen before include "Financially comfortable" , "Rich; fortunate" , "Moderately rich" , "Fairly rich" , "Quite wealthy" .)